DOI: 10.1002/chem.201500860
Full Paper
& Electron Donation
Moving toward Ylide-Stabilized Carbenes Bitupon Borthakur and Ashwini K. Phukan*[a] Abstract: The effect of ylide substitution at the a position to the carbene carbon (Cc) atom on the stability and s-donating ability of a number of cyclic carbenes has been studied theoretically. The stabilities of all of the carbenes were investigated from an evaluation of their singlet–triplet energy gaps and stabilization energies. All carbenes were found to have a stable singlet state. The energy of the s-
Introduction The neutral divalent species carbene can exhibit two spin states that depend on the nature of occupancy of the two nonbonding electrons. A singlet state arises if the two nonbonding electrons reside within the same orbital with antiparallel spin, whereas two nonbonding electrons residing in two mutually perpendicular orbitals with parallel spin give rise to a triplet state. The parent carbene (:CH2) possesses a triplet ground state,[1] but carbenes with stable singlet states have also been reported. A signature example of a carbene with a stable singlet state is the N-heterocyclic carbene (NHC);[2] these have been widely used for more than two decades in many organic transformations and transition-metal-mediated catalysis.[3–8] The remarkable stability of the singlet state of NHCs is traced to the extensive p donation from the lone pairs of the neighboring nitrogen atoms (NLP) to the formally vacant p orbital of the carbene center (NLP !CPp).[9] The stronger the NLP !CPp interaction is, the higher the stability of the singlet state will be. Thus, the presence of a heteroatom next to the carbene center was considered as a necessary criterion for obtaining stable singlet carbenes. On this basis, six years after the isolation of NHCs, a new class of isolable cyclic carbenes, the thiazolylidenes, was prepared with nitrogen and sulfur atoms as the heteroatoms.[10] Similarly, in 2005, a P-heterocyclic carbene (PHC) was isolated.[11] However, the singlet–triplet separations of the thiazolylidenes and the PHC were found to be lower than those of NHCs.[12] This difference in the singlet–triplet separations arises from the inferior p-donating ability of sulfur and phosphorus relative to that of nitrogen. However, the isolation of a cyclopropenylidene derivative by Bertrand [a] B. Borthakur, Prof. A. K. Phukan Department of Chemical Sciences, Tezpur University Napam—784 028, Assam (India) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500860. Chem. Eur. J. 2015, 21, 11603 – 11609
symmetric lone-pair orbital at the Cc atom increases as a result of the introduction of ylide centers near to the Cc atom. This indicates an enhanced s-donating ability of the ylide-containing carbenes. The calculated carbonyl-stretching frequencies of the corresponding rhodium complexes, proton affinities, and nucleophilicity index values correlate well with the s basicity of the carbenes.
and co-workers indicated the possibility of a stable cyclic carbene without the presence of a heteroatom directly bonded to the carbene center.[13] NHCs have been found to have very good s-donating ability, as well as considerable p-accepting ability.[14, 15] The enhanced s-donating ability and steric bulk of NHCs allows them to replace phosphines as the ligands in many transition-metal catalysts, like the Grubbs second-generation olefin-metathesis catalyst.[16, 17] In 2005, Bertrand and co-workers isolated a cyclic (alkyl) (amino) carbene (CAAC), which possesses better s basicity and p acidity than the NHCs.[18, 19] The PdII and AuI complexes of CAAC have shown surprising and novel catalytic behavior.[20, 21] The more nucleophilic and electrophilic character of CAAC was found to be advantageous in the activation of small molecules like H2, NH3, and P4.[22, 23] This has opened up a new possibility for the design of stable singlet carbenes with better s-donating ability. The electron-donating ability of the carbene ligand has a significant effect on the catalytic activity of the corresponding transition-metal complexes. Hence, the design and synthesis of carbene frameworks with enhanced electron-donating abilities may help to develop novel catalysts for various applications. One way to enhance the s basicity of carbenes is by skeletal substitution or placement of appropriate substituents at the atoms adjacent to the carbene center. Another way is to introduce electron-donating phosphorus and sulfur ylide centers into the ring framework.[24] The installation of ylide centers next to the carbenic carbon atom is expected not only to enhance the s-donating ability but also to stabilize the carbene molecule. The enhanced s-donating ability results from a smaller inductive effect of the ylide groups relative to the amino groups, whereas the stabilization comes from effective p donation of the ylide carbanion to the carbene center. To the best of our knowledge, apart from the seminal works of Kawashima and Fìrstner in 2008,[24] there has been no systematic study, either theoretical or experimental, toward the exploration of this novel class of ylide-stabilized carbenes. In this work, we
11603
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
Scheme 1. Schematic representation of the range of carbenes considered in this study.[31] Dipp: 2,6-diisopropylphenyl.
make an effort towards contributing to the field of phosphorus ylide stabilized carbenes (Scheme 1). It was found previously that the electron-donating abilities of sulfur ylide stabilized carbenes are lower than those with phosphorus ylide substitution,[24c] so we did not consider sulfur ylide stabilized carbenes in the present study. We have considered both five- and sixmembered parent carbenes, as well as their skeletally substituted derivatives, to understand the effect of ylide substitution on the electron-donating ability and stability. Furthermore, this study also includes hitherto-unknown ylide-stabilized boronsubstituted carbenes.
Results and Discussion
Computational details Density functional theory calculations were performed to optimize all of the molecules with the hybrid PBE0 exchange-correlation functional.[25] We used the 6-31 + G* basis set for maingroup elements and the SDD basis set with the Stuttgart–Dresden relativistic effective core potential for the rhodium atom.[26] To check the reliability of the basis set and the functional, the singlet–triplet separation of a representative molecule was evaluated by employing four different basis sets and functionals (Table S1 in the Supporting Information). There was no appreciable change in the values of the singlet–triplet energy gaps (DES–T), which indicated that the basis set and the functional used in our study were quite adequate to predict the singlet–triplet gap and other properties considered in this study. This level of theory was found to be adequate in dealing with similar systems, as reported recently.[27, 28] Furthermore, we have calculated the 13C NMR chemical shifts of the experimentally known molecules (1, 14, 17, and 20) and found that the level of theory used in this work could successfully reproduce Chem. Eur. J. 2015, 21, 11603 – 11609
the observed NMR shifts (Table S2 in the Supporting Information). Frequency calculations were performed at the same level of theory to characterize the nature of the stationary point. All structures were found to be minima on the potential energy surface with real frequencies. Natural-bonding analyses were performed with the natural-bond-orbital (NBO) partitioning scheme,[29] as implemented in the Gaussian 03 suite of programs.[30] We calculated the nucleophilicity index with reference to tetracyanoethylene (TCNE), which was optimized at the same level of theory.
www.chemeurj.org
Geometries The central rings of 2, 3, 5–7, 9, 10, 12, 13, and 20–22 (Scheme 1) are found to have a perfectly planar structure in the optimized stable singlet state and a slightly distorted structure for others (Figure S1 in the Supporting Information). The important geometrical parameters of 1–22 in the singlet state are listed in Table 1. All of the carbene molecules (except 11) have a wider E¢Cc¢ E (Cc : central carbenic carbon atom; E: C or N) bond angle in the triplet state than in the singlet state. Two different Cc¢E bond lengths are obtained for the molecules with an unsymmetrical skeleton (Table 1). Our calculated Cc¢E bond lengths and E¢Cc¢E bond angles for 1, 14, 17, and 20 are in excellent agreement with the experimentally observed values (Table 1). A comparison of the geometrical parameters of 1–3 indicates that there is a significant change in the central E¢Cc¢E bond angle as a result of changes in the backbone of the ring framework. There is also a marginal change in the Cc¢E bond
11604
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Singlet–triplet separation and thermodynamic stabilities
Table 1. PBE0/6-31 + G*-calculated Cc¢E bond lengths (r1/r2) and E¢Cc¢E bond angles of the singlet-state geometry of 1–22 (E: C or N).[a]
Molecule
r1/r2 [æ]
a(E¢Cc¢E) [8]
Ref.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1.519/1.311 (1.516/1.315) 1.524/1.325 1.515/1.326 1.405/1.359 1.419/1.375 1.404/1.387 1.422/1.355 1.529/1.362 1.533/1.380 1.522/1.380 1.414/1.415 1.425/1.425 1.409/1.433 1.345/1.345 (1.346) 1.401/1.350 1.404/1.404 1.371/1.371 (1.352/1.404) 1.429/1.376 1.439/1.439 1.360/1.360 (1.366/1.363) 1.408/1.368 1.418/1.418
106.3 103.6 101.8 105.2 101.2 98.3 99.9 105.0 102.5 100.6 104.0 99.9 97.1 115.6 116.0 116.3 110.8 112.4 111.5 114.9 115.4 115.7
[18]
(106.5)
As a first approximation, the stability of these molecules can be judged from their respective singlet–triplet energy gaps (DES–T).[12] In general, the stability of the singlet state increases with an increase in the value of DES–T. The calculated DES–T values are given in Table 2.
Table 2. PBE0/6-31 + G*-calculated singlet–triplet separations (DES–T) and stabilization energies (SE) of 1–22.
(114.6)
[32]
(108.4)
[33]
(114.4)
[34]
[a] Experimental values are given within parenthesis.
lengths. Similar results are obtained for 4–6, 8–10, and 11–13. Replacement of the two methyl groups attached to the carbon atom in the a position to the Cc atom in 1–3 by the ylide centers in 4–7 changes the Cc¢E bond lengths significantly. The Cc¢C (r1) bond lengths are found to decrease, whereas there is an increase in the Cc¢N (r2) bond lengths. This indicates an increase in electron delocalization between the formally vacant p orbital of the Cc atom and the adjacent carbon atom (Ca) but a decrease in delocalization from the nitrogen lone pair (NLP) to the formally vacant p orbital of the Cc atom. This is also evident from the increase in occupancy of the NLP in 4–7 (1.682, 1.578, 1.572, and 1.561, respectively) relative to that in 1–3 (1.551, 1.540, and 1.542, respectively). The E¢Cc¢E bond angles also change appreciably. There is an increase in the Cc¢E (both r1 and r2) bond lengths and a slight decrease in the E¢Cc¢E bond angles upon replacement of the amino groups of 1–3 with the ylide centers in 8–10. However, molecules 11–13, in which two ylide groups are introduced, were found to have smaller E¢Cc¢E bond angles than the parent compounds with identical backbones. Comparison of the Cc¢E bond lengths of 4–6 with those of 11–13 indicates that there is an increase in the Cc¢E (both r1 and r2) bond lengths as a result of replacement of the amino groups with ylide centers. However, there is a decrease in the r1 value and an increase in the r2 value upon replacement of the two methyl groups in 8–10 with the ylide centers in 11–13. In the case of the six-membered NHCs with a saturated backbone (14–16), the Cc¢E (both r1 and r2) bond lengths are found to increase upon replacement of the amino groups with ylide centers. Similar results are also obtained for boron-substituted five- (17–19) and six- (20–22) membered NHCs. However, in all three cases, the change in the central E¢Cc¢E bond angle is not significant. Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
Molecule
DES–T [kcal mol¢1]
SE [kcal mol¢1]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
43.9 40.3 39.6 55.3 59.4 63.5 61.7 40.4 37.1 36.8 57.5 59.9 64.2 58.0 50.3 50.4 43.6 35.3 31.7 42.2 39.6 40.6
79.4 72.6 69.8 103.6 114.5 114.6 113.9 84.7 77.3 74.9 105.8 114.3 115.8 101.9 97.7 107.3 82.3 86.9 90.4 87.7 86.4 95.8
It is evident from Table 2 that all of the molecules have a stable singlet ground state. The singlet state of 1 with a saturated backbone is slightly more stable than 2 with an unsaturated backbone. We did not observe any dramatic change in the stability of the singlet states as a result of the introduction of a nitrogen atom into the olefinic backbone. For example, the DES–T values of 2 and 3 (40.3 and 39.6 kcal mol¢1, respectively) and those of 9 and 10 (37.1 and 36.8 kcal mol¢1, respectively) are comparable. There is a significant increase in the DES–T values as a result of introduction of the more-electrondonating ylide groups (in 4–7) in place of the two methyl groups (in 1–3). On the other hand, replacement of the amino groups (in 1–3) by ylidic ones (in 8–10) decreases the DES–T values, albeit marginally. A comparison of the DES–T values of 8–10 with those of 11–13 shows that the introduction of one more ylide group dramatically enhances the stability. However, a comparison of 4–6 with 11–13 indicates that there is no significant increase in the DES–T values as a result of the replacement of the amine group with one more ylide group. Among the saturated six-membered NHCs (14–16), the parent carbene is found to have a higher DES–T value than those compounds with phosphorus ylide groups at the a position with respect to the carbene carbon atom. Similarly, lower singlet–triplet separation is obtained for both five- and six-membered boron-sub-
11605
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper stituted NHCs with ylide groups at the a position to the Cc atom relative to the parent compounds. The changes are more appreciable for the five-membered ones. The stabilization energies (SE) of 1–22 were evaluated by using Equation (1) and these values were further used to obtain a measure of the thermodynamic stabilities of all of the molecules. It may be noted that the calculated stabilization energies are the same as indirect hydrogenation energies. The calculated values of the stabilization energies are listed in Table 2.
Figure 1. Plot of the energies of the s-symmetric lone-pair orbitals (Es) concentrated on the central carbon atom of 1–22.
The stabilization energies of carbenes 1–13 follow an almost similar trend to the DES–T values. However, for molecules 14– 22, the stabilization energies do not go in parallel with the DES–T values. For example, the DES–T values of 17–19 are found to decrease as a result of the introduction of ylide groups, whereas the stabilization energies follow a reverse order. We obtained a reasonable correlation (R2 = 0.82, with omission of the point corresponding to 19; Figure S2 in the Supporting Information) between the DES–T and SE values and both of these values predict 13 to be the thermodynamically most stable molecule. The singlet–triplet separation and stabilization energies of 4–6 and 11–13 are found to be comparable with the theoretically modeled experimentally known carbenes 1 and 7. Similar comparisons can be made between the six-membered carbenes 15–16 and the boron-substituted carbenes 18–19 and 21–22 with the experimentally known carbenes 14, 17, and 20, respecively.
nating ability. A similar increase in electron-donating ability is obtained for six-membered NHCs with a saturated backbone (14–16) and for both five- (17–19) and six- (20–22) membered boron-substituted NHCs. Out of all of the molecules, 19 is found to have the highest s-donating ability. In order to get a better understanding of the s-donating ability of the carbenes considered for this study, we have analyzed the hybridization of the s-symmetric lone-pair orbital (LPs) at the Cc atom, along with the percentage of s-character (Table 3). The Es values increase with a decrease in the s-character of the lone pair at the Cc atom. Thus, molecules with lower s-character in the LPs are found to have better s-donating abilities. In fact, we obtained a good correlation (R2 = 0.79) between the energies and the percentage of s-character of the s-symmetric lone-pair orbitals at the Cc atom (Figure S3 in the
Ligand properties Both theoretical and experimental studies suggested that carbenes are very good s donors.[14, 15] The s-donating ability of carbene depends on the nature and energy of the s- symmetric lone-pair orbital (Es) concentrated at the Cc atom.[35] The higher the energy of this frontier orbital, the higher the donating ability will be. We have performed NBO[29] analysis in order to determine the energies of these s-donating frontier orbitals. The calculated Es values are listed in Table 3 and are graphically represented in Figure 1. Both Table 3 and Figure 1 indicate that there is a marginal decrease (except for 12) in the s-donating ability of carbenes 1–13 as a result of changes in the backbone. It is apparent from Figure 1 that replacement of the two methyl groups attached to the a-carbon atom with respect to the Cc atom in 1– 3 by the more-electron-releasing ylide groups (in 4–7) dramatically lifts the Es values and thereby significantly enhances the basicity of 4–7. Similarly, replacement of the amino groups of 1–3 with ylide groups (in 8–10) significantly increases the sdonating abilities. The introduction of two ylide centers near to the Cc atom (in 11–13) is found to further increase the s-doChem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
11606
Table 3. PBE0/6-31 + G*-calculated energies (Es), hybridization (% s character) of the s-symmetric lone-pair orbital concentrated at the carbene carbon atom and the average carbonyl-stretching frequencies (nCO) of L¢ Rh(CO)2Cl (L: 1–22) complexes.
Molecule
Es [eV]
Hybridization (% s character)
nCO (avg.) [cm¢1]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
¢5.5 ¢5.8 ¢6.2 ¢4.8 ¢5.0 ¢5.2 ¢5.3 ¢4.5 ¢4.6 ¢4.9 ¢4.1 ¢4.0 ¢4.2 ¢5.4 ¢4.3 ¢3.9 ¢5.3 ¢4.4 ¢3.5 ¢5.1 ¢4.3 ¢3.8
sp1.20 (45.4) sp1.09 (47.7) sp1.09 (47.8) sp1.48 (40.3) sp1.29 (43.7) sp1.26 (44.3) sp1.30 (43.5) sp1.70 (36.8) sp1.44 (41.0) sp1.44 (41.0) sp1.98 (33.5) sp1.68 (37.3) sp1.65 (37.7) sp1.23 (44.8) sp1.79 (35.8) sp2.60 (27.7) sp1.03 (49.1) sp1.49 (40.1) sp1.81 (35.6) sp1.11 (47.3) sp1.63 (37.9) sp2.25 (30.8)
2131 2136 2141 2120 2125 2129 2131 2122 2123 2129 2112 2112 2112 2136 2122 2110 2136 2122 2110 2131 2120 2101
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper ability (Table 3) and, consequently, the nCO value of the respective rhodium complex is found to be the highest. Similarly, for molecules like 16, 19, and 22, which are better s donors, the carbonyl-stretching frequencies are significantly lower than those of others. We have obtained a nice correlation (R2 = 0.88) between the Es and nCO values (Figure 3).
Figure 2. Correlation plot between the proton affinities [kcal mol¢1] and energies of the s-symmetric lone pair orbitals concentrated on the carbene carbon (Es [eV]) of 1–22.
Supporting Information). The basicity of these carbene molecules can also be gauged from an evaluation of their respective proton affinities.[36] The higher the basicity, the higher the proton affinity (PA) will be. Indeed, we obtained a nice correlation (R2 = 0.90) between the computed Es and PA values (Figure 2 and Table S3 in the Supporting Information). In agreement with their enhanced s-donating abilities, the PA values of the bis-ylide-containing carbenes are found to be significantly higher (> 300 kcal mol¢1) than others. The carbonyl-stretching frequencies (nCO) of the corresponding metal carbonyl complexes can be used as an important tool to measure the s-donating abilities of the carbene ligands.[37] The higher the s basicity of a carbene ligand, the higher the electron density at the metal center will be and, consequently, the higher the extent of back donation from the metal center to the CO antibonding orbital will be (Scheme 2), which will thereby shift the carbonyl-stretching frequencies (nCO) to lower values.
Figure 3. Correlation plot between the energies of the s-symmetric lonepair orbitals concentrated at the central carbene carbon atom (Es [eV]) and the nCO values [cm¢1] of the L¢Rh(CO)2Cl complexes (L: 1–22).
Nucleophilicity index The donor strength of all of the carbene molecules was further assessed by evaluating their nucleophilicity indices. The nucleophilicity index is calculated by using the equation N = EHOMO¢EHOMO(TCNE), with tetracyanoethylene (TCNE) considered as the reference.[38] The calculated values of the nucleophilicity indices are listed in Table 4.
Table 4. PBE0/6-31 + G*-calculated nucleophilicity indices (N) of 1–22.
Scheme 2. Schematic representation of the back-bonding interaction between the metal center and the CO p* orbital.
To get a quantitative measure of the electron-donating ability of the carbene ligands under consideration, we have optimized square-planar complexes of rhodium, L¢Rh(CO)2Cl (L: 1– 22) with the two carbonyl groups in the cis positions. The calculated values of nCO for all of the complexes are given in Table 3. The calculated average nCO values are found to vary as a function of the relative s basicity of the carbene ligands. The computed nCO values of the metal complexes containing ylidesubstituted carbenes are found to be appreciably lower than those of others. This indicates significantly enhanced electrondonating abilities of the ylide-containing carbenes. Among all of the molecules, 3 is found to have the lowest s-donating Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
Molecule
N
Molecule
N
1 2 3 4 5 6 7 8 9 10 11
4.1 3.7 3.4 4.8 4.6 4.4 4.3 5.1 5.0 4.7 5.9
12 13 14 15 16 17 18 19 20 21 22
5.6 5.4 4.2 5.3 5.9 4.3 5.2 6.1 4.5 5.3 5.8
The nucleophilicity indices of the carbene molecules are found to be in the range of 3.4–6.1 eV. These values show a similar trend to the respective Es values. The values of the nucleophilicity indices support the increase in s-donating ability of carbenes as a result of the introduction of ylide centers into the ring framework. The N values are found to be highest for carbenes with two ylide groups adjacent to the Cc atom. The highest and lowest s basicities are obtained for 19 and 3 (Table 3), respectively, which is in agreement with their respective N values. Indeed, we have obtained an excellent correlation (R2 = 0.98) between the Es and N values (Figure 4).
11607
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Conclusion
Figure 4. Correlation plot between Es [eV] and N [eV] for 1–22.
Recently, Gandon and co-workers have shown that the nucleophilicity of a carbene ligand may be correlated with the degree of gallium pyramidalization (qGa ; qGa = 3608¢Cl¢Ga¢ Cl) in the carbene adducts of GaCl3.[39] By following this approach, we have calculated the GaCl3 adducts of 1–22 and obtained a good correlation (R2 = 0.85) between the nucleophilicity index and qGa values, as well as a nice correlation (R2 = 0.90) between the Ga¢Cc bond dissociation energies and bond lengths (Figure 5 and Table S3 in the Supporting Information). Furthermore, we have also obtained a nice correlation (R2 = 0.92) between the calculated proton affinity and the degree of Ga pyramidalization (Figure S4 in the Supporting Information).
Density functional theory calculations have been carried out on a number of cyclic carbene molecules with or without heteroatoms in the ring framework. All of these molecules exhibit a stable singlet ground state. The introduction of ylide centers into the ring framework was found to dramatically enhance the s-donating ability. Furthermore, for majority of the molecules, ylide substitution leads to a significant increase in stability. The carbonyl-stretching frequencies of the corresponding metal complexes and the nucleophilicity index values were found to correlate well with the s basicity of these carbenes. The calculated proton affinity values and the extent of Ga pyramidalization were also in excellent agreement with the s basicity of these carbenes. We hope that our study will trigger new experimental studies towards the exploration of novel ylide-stabilized carbenes.
Acknowledgements A.K.P. thanks Tezpur University for financial assistance and Prof. Holger Braunschweig for kindly granting access to the computational facilities of his research group. B.B. thanks Tezpur University for an institutional fellowship. Keywords: boron · carbenes · electron donation · rhodium · ylides
Figure 5. Correlation plots between: a) the pyramidalization angle at the gallium atom (qGa in degrees) of the GaCl3 adducts and the nucleophilicity indices (N), and b) the Ga¢Cc bond dissociation energies (BDE [kcal mol¢1]) and the Ga¢Cc bond lengths [æ] of the GaCl3 adducts of 1–22. Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
[1] a) K. Balasubramanian, A. D. McLean, J. Chem. Phys. 1986, 85, 5117; b) W. D. Allen, H. F. Schaefer, Chem. Phys. 1986, 108, 243. [2] A. J. Arduengo, III, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113, 361. [3] W. A. Herrmann, M. Elison, J. Fischer, C. Kçcher, G. R. J. Artus, Angew. Chem. Int. Ed. Engl. 1995, 34, 2371; Angew. Chem. 1995, 107, 2602. [4] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107, 5606. [5] F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122; Angew. Chem. 2008, 120, 3166. [6] S. Dez-Gonzlez, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109, 3612. [7] K. V. S. Ranganath, J. Kloesges, A. H. Schfer, F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 7786; Angew. Chem. 2010, 122, 7952. [8] a) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 2014, 510, 485; b) D. K. Roy, B. Mondal, R. S. Anju, S. Ghosh, Chem. Eur. J. 2015, 21, 3640. [9] A. J. Arduengo, III, H. V. R. Dias, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1992, 114, 5530. [10] A. J. Arduengo, III, J. R. Goerlich, W. J. Marshall, Liebigs Ann. 1997, 365. [11] D. Martin, A. Baceiredo, H. Gornitzka, W. W. Schoeller, G. Bertrand, Angew. Chem. Int. Ed. 2005, 44, 1700; Angew. Chem. 2005, 117, 1728. [12] a) D. Bourissou, O. Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev. 2000, 100, 39; b) S. Gronert, J. R. Keeffe, R. A. More O’Ferrall, J. Am. Chem. Soc. 2011, 133, 3381. [13] V. Lavallo, Y. Canac, B. Donnadieu, W. W. Schoeller, G. Bertrand, Science 2006, 312, 722. [14] a) C. Boehme, G. Frenking, J. Am. Chem. Soc. 1996, 118, 2039; b) A. A. Tukov, A. T. Normand, M. S. Nechaev, Dalton Trans. 2009, 7015; c) P. Bazinet, G. A. P. Yap, D. S. Richeson, J. Am. Chem. Soc. 2003, 125, 13314; d) N. Kuhn, A. Al-Sheikh, Coord. Chem. Rev. 2005, 249, 829; e) V. Nair, S. Bindu, V. Sreekumar, Angew. Chem. Int. Ed. 2004, 43, 5130; Angew. Chem. 2004, 116, 5240. [15] a) M. Alcarazo, T. Stork, A. Anoop, W. Thiel, A. Fìrstner, Angew. Chem. Int. Ed. 2010, 49, 2542; Angew. Chem. 2010, 122, 2596; b) H. Seo, B. P. Roberts, K. A. Abboud, K. M. Merz, S. Hong, Org. Lett. 2010, 12, 4860;
11608
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
[16] [17] [18]
[19] [20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
c) T. W. Hudnall, C. W. Bielawski, J. Am. Chem. Soc. 2009, 131, 16039; d) R. Tonner, G. Heydenrych, G. Frenking, Chem. Asian J. 2007, 2, 1555. a) M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1, 953; b) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18. J. Huang, E. D. Stevens, S. P. Nolan, J. L. Petersen, J. Am. Chem. Soc. 1999, 121, 2674. a) V. Lavallo, Y. Canac, C. Prsang, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2005, 44, 5705; Angew. Chem. 2005, 117, 5851; b) V. Lavallo, Y. Canac, A. DeHope, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2005, 44, 7236; Angew. Chem. 2005, 117, 7402. M. Soleilhavoup, G. Bertrand, Acc. Chem. Res. 2015, 48, 256 and references therein. N. Holzmann, D. M. Andrada, G. Frenking, J. Organomet. Chem. 2015, DOI: 10.1016/j.jorganchem.2015.03.029. a) V. Lavallo, G. D. Frey, S. Kousar, B. Donnadieu, G. Bertrand, Proc. Natl. Acad. Sci. USA 2007, 104, 13569; b) G. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G. Bertrand, Science 2007, 316, 439. P. P. Samuel, K. C. Mondal, H. W. Roesky, M. Hermann, G. Frenking, S. Demeshko, F. Meyer, A. C. Stìckl, J. H. Christian, N. S. Dalal, L. Ungur, L. F. Chibotaru, K. Prçpper, A. Meents, B. Dittrich, Angew. Chem. Int. Ed. 2013, 52, 11817; Angew. Chem. 2013, 125, 12033. a) J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2007, 46, 7052; Angew. Chem. 2007, 119, 7182; b) J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand, J. Am. Chem. Soc. 2007, 129, 14180; c) O. Back, G. Kuchenbeiser, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2009, 48, 5530; Angew. Chem. 2009, 121, 5638. a) S. Nakafuji, J. Kobayashi, T. Kawashima, Angew. Chem. Int. Ed. 2008, 47, 1141; Angew. Chem. 2008, 120, 1157; b) M. Asay, B. Donnadieu, A. Baceiredo, M. Soleilhavoup, G. Bertrand, Inorg. Chem. 2008, 47, 3949; c) A. Fìrstner, M. Alcarazo, K. Radkowski, C. W. Lehmann, Angew. Chem. Int. Ed. 2008, 47, 8302; Angew. Chem. 2008, 120, 8426; d) J. Kobayashi, S. Nakafuji, A. Yatabe, T. Kawashima, Chem. Commun. 2008, 6233. a) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865; b) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1997, 78, 1396; c) J. P. Perdew, K. Burke, M. Ernzerhof, J. Chem. Phys. 1996, 105, 9982; d) M. Ernzerhof, G. E. Scuseria, J. Chem. Phys. 1999, 110, 5029. a) M. Dolg, U. Wedig, H. Stoll, H. Preuss, J. Chem. Phys. 1987, 86, 866; b) D. Andrae, U. Hausserermann, M. Dolg, U. Wedig, H. Stoll, H. Preuss, Theor. Chim. Acta. 1990, 77, 123; c) A. Alkauskas, A. Baratoff, C. Bruder, J. Phys. Chem. A 2004, 108, 6863. a) A. K. Phukan, A. K. Guha, S. Sarmah, R. D. Dewhurst, J. Org. Chem. 2013, 78, 11032; b) A. K. Phukan, A. K. Guha, S. Sarmah, Organometallics 2013, 32, 3238; c) A. K. Guha, A. K. Phukan, J. Org. Chem. 2014, 79, 3830; d) B. Borthakur, T. Rahman, A. K. Phukan, J. Org. Chem. 2014, 79, 10801. a) A. A. Fokin, L. V. Chernish, P. A. Gunchenko, E. Y. Tikhonchuk, H. Hausmann, M. Serafin, J. E. P. Dahl, R. M. K. Carlson, P. R. Schreiner, J. Am. Chem. Soc. 2012, 134, 13641; b) B. D. Rekken, T. M. Brown, J. C. Fettinger, F. Lips, H. M. Tuononen, R. H. Herber, P. P. Power, J. Am. Chem. Soc. 2013, 135, 10134; c) C. Hering, A. Schulz, A. Villinger, Angew. Chem. Int. Ed. 2012, 51, 6241; Angew. Chem. 2012, 124, 6345.
Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
[29] a) E. D. Glendening, A. E. Reed, J. E. Carpenter, F. Weinhold, NBO Program 3.1, W. T. Madison: 1988; b) A. E. Reed, F. Weinhold, L. A. Curtiss, Chem. Rev. 1988, 88, 899. [30] Gaussian 03, Revision D.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. J. Salvador, J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al- Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 2003. [31] The ylide-stabilized carbenes can be depicted by a resonance structure consisting of a double-bonded form, as shown in Scheme 1, and a zwitterionic form, in which positive and negative charges are placed at the phosphorus and carbon atoms, respectively. An example is shown below:
[32] M. Iglesias, D. J. Beetstra, J. C. Knight, L. Ooi, A. Stasch, S. Coles, L. Male, M. B. Hursthouse, K. J. Cavel, A. Dervisi, I. A. Fallis, Organometallics 2008, 27, 3279. [33] K. E. Krahulic, G. D. Enright, M. Parvez, R. Roesler, J. Am. Chem. Soc. 2005, 127, 4142. [34] C. Prsang, B. Donnadieu, G. Bertrand, J. Am. Chem. Soc. 2005, 127, 10182. [35] a) H. Jacobsen, A. Correa, C. Poater, C. Costabile, L. Cavallo, Coord. Chem. Rev. 2009, 253, 687; b) U. Radius, F. M. Bickelhaupt, Coord. Chem. Rev. 2009, 253, 678. [36] R. Tonner, G. Heydenrych, G. Frenking, ChemPhysChem 2008, 9, 1474. [37] a) A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller, R. H. Crabtree, Organometallics 2003, 22, 1663; b) D. G. Gusev, Organometallics 2009, 28, 763. [38] L. R. Domingo, E. Chamorro, P. Perez, J. Org. Chem. 2008, 73, 4615. [39] A. El-Hellani, J. Monot, S. Tang, R. Guillot, C. Bour, V. Gandon, Inorg. Chem. 2013, 52, 11493. Received: March 3, 2015 Published online on June 26, 2015
11609
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
& Electron Donation
Moving toward Ylide-Stabilized Carbenes Bitupon Borthakur and Ashwini K. Phukan*[a] Abstract: The effect of ylide substitution at the a position to the carbene carbon (Cc) atom on the stability and s-donating ability of a number of cyclic carbenes has been studied theoretically. The stabilities of all of the carbenes were investigated from an evaluation of their singlet–triplet energy gaps and stabilization energies. All carbenes were found to have a stable singlet state. The energy of the s-
Introduction The neutral divalent species carbene can exhibit two spin states that depend on the nature of occupancy of the two nonbonding electrons. A singlet state arises if the two nonbonding electrons reside within the same orbital with antiparallel spin, whereas two nonbonding electrons residing in two mutually perpendicular orbitals with parallel spin give rise to a triplet state. The parent carbene (:CH2) possesses a triplet ground state,[1] but carbenes with stable singlet states have also been reported. A signature example of a carbene with a stable singlet state is the N-heterocyclic carbene (NHC);[2] these have been widely used for more than two decades in many organic transformations and transition-metal-mediated catalysis.[3–8] The remarkable stability of the singlet state of NHCs is traced to the extensive p donation from the lone pairs of the neighboring nitrogen atoms (NLP) to the formally vacant p orbital of the carbene center (NLP !CPp).[9] The stronger the NLP !CPp interaction is, the higher the stability of the singlet state will be. Thus, the presence of a heteroatom next to the carbene center was considered as a necessary criterion for obtaining stable singlet carbenes. On this basis, six years after the isolation of NHCs, a new class of isolable cyclic carbenes, the thiazolylidenes, was prepared with nitrogen and sulfur atoms as the heteroatoms.[10] Similarly, in 2005, a P-heterocyclic carbene (PHC) was isolated.[11] However, the singlet–triplet separations of the thiazolylidenes and the PHC were found to be lower than those of NHCs.[12] This difference in the singlet–triplet separations arises from the inferior p-donating ability of sulfur and phosphorus relative to that of nitrogen. However, the isolation of a cyclopropenylidene derivative by Bertrand [a] B. Borthakur, Prof. A. K. Phukan Department of Chemical Sciences, Tezpur University Napam—784 028, Assam (India) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201500860. Chem. Eur. J. 2015, 21, 11603 – 11609
symmetric lone-pair orbital at the Cc atom increases as a result of the introduction of ylide centers near to the Cc atom. This indicates an enhanced s-donating ability of the ylide-containing carbenes. The calculated carbonyl-stretching frequencies of the corresponding rhodium complexes, proton affinities, and nucleophilicity index values correlate well with the s basicity of the carbenes.
and co-workers indicated the possibility of a stable cyclic carbene without the presence of a heteroatom directly bonded to the carbene center.[13] NHCs have been found to have very good s-donating ability, as well as considerable p-accepting ability.[14, 15] The enhanced s-donating ability and steric bulk of NHCs allows them to replace phosphines as the ligands in many transition-metal catalysts, like the Grubbs second-generation olefin-metathesis catalyst.[16, 17] In 2005, Bertrand and co-workers isolated a cyclic (alkyl) (amino) carbene (CAAC), which possesses better s basicity and p acidity than the NHCs.[18, 19] The PdII and AuI complexes of CAAC have shown surprising and novel catalytic behavior.[20, 21] The more nucleophilic and electrophilic character of CAAC was found to be advantageous in the activation of small molecules like H2, NH3, and P4.[22, 23] This has opened up a new possibility for the design of stable singlet carbenes with better s-donating ability. The electron-donating ability of the carbene ligand has a significant effect on the catalytic activity of the corresponding transition-metal complexes. Hence, the design and synthesis of carbene frameworks with enhanced electron-donating abilities may help to develop novel catalysts for various applications. One way to enhance the s basicity of carbenes is by skeletal substitution or placement of appropriate substituents at the atoms adjacent to the carbene center. Another way is to introduce electron-donating phosphorus and sulfur ylide centers into the ring framework.[24] The installation of ylide centers next to the carbenic carbon atom is expected not only to enhance the s-donating ability but also to stabilize the carbene molecule. The enhanced s-donating ability results from a smaller inductive effect of the ylide groups relative to the amino groups, whereas the stabilization comes from effective p donation of the ylide carbanion to the carbene center. To the best of our knowledge, apart from the seminal works of Kawashima and Fìrstner in 2008,[24] there has been no systematic study, either theoretical or experimental, toward the exploration of this novel class of ylide-stabilized carbenes. In this work, we
11603
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
Scheme 1. Schematic representation of the range of carbenes considered in this study.[31] Dipp: 2,6-diisopropylphenyl.
make an effort towards contributing to the field of phosphorus ylide stabilized carbenes (Scheme 1). It was found previously that the electron-donating abilities of sulfur ylide stabilized carbenes are lower than those with phosphorus ylide substitution,[24c] so we did not consider sulfur ylide stabilized carbenes in the present study. We have considered both five- and sixmembered parent carbenes, as well as their skeletally substituted derivatives, to understand the effect of ylide substitution on the electron-donating ability and stability. Furthermore, this study also includes hitherto-unknown ylide-stabilized boronsubstituted carbenes.
Results and Discussion
Computational details Density functional theory calculations were performed to optimize all of the molecules with the hybrid PBE0 exchange-correlation functional.[25] We used the 6-31 + G* basis set for maingroup elements and the SDD basis set with the Stuttgart–Dresden relativistic effective core potential for the rhodium atom.[26] To check the reliability of the basis set and the functional, the singlet–triplet separation of a representative molecule was evaluated by employing four different basis sets and functionals (Table S1 in the Supporting Information). There was no appreciable change in the values of the singlet–triplet energy gaps (DES–T), which indicated that the basis set and the functional used in our study were quite adequate to predict the singlet–triplet gap and other properties considered in this study. This level of theory was found to be adequate in dealing with similar systems, as reported recently.[27, 28] Furthermore, we have calculated the 13C NMR chemical shifts of the experimentally known molecules (1, 14, 17, and 20) and found that the level of theory used in this work could successfully reproduce Chem. Eur. J. 2015, 21, 11603 – 11609
the observed NMR shifts (Table S2 in the Supporting Information). Frequency calculations were performed at the same level of theory to characterize the nature of the stationary point. All structures were found to be minima on the potential energy surface with real frequencies. Natural-bonding analyses were performed with the natural-bond-orbital (NBO) partitioning scheme,[29] as implemented in the Gaussian 03 suite of programs.[30] We calculated the nucleophilicity index with reference to tetracyanoethylene (TCNE), which was optimized at the same level of theory.
www.chemeurj.org
Geometries The central rings of 2, 3, 5–7, 9, 10, 12, 13, and 20–22 (Scheme 1) are found to have a perfectly planar structure in the optimized stable singlet state and a slightly distorted structure for others (Figure S1 in the Supporting Information). The important geometrical parameters of 1–22 in the singlet state are listed in Table 1. All of the carbene molecules (except 11) have a wider E¢Cc¢ E (Cc : central carbenic carbon atom; E: C or N) bond angle in the triplet state than in the singlet state. Two different Cc¢E bond lengths are obtained for the molecules with an unsymmetrical skeleton (Table 1). Our calculated Cc¢E bond lengths and E¢Cc¢E bond angles for 1, 14, 17, and 20 are in excellent agreement with the experimentally observed values (Table 1). A comparison of the geometrical parameters of 1–3 indicates that there is a significant change in the central E¢Cc¢E bond angle as a result of changes in the backbone of the ring framework. There is also a marginal change in the Cc¢E bond
11604
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Singlet–triplet separation and thermodynamic stabilities
Table 1. PBE0/6-31 + G*-calculated Cc¢E bond lengths (r1/r2) and E¢Cc¢E bond angles of the singlet-state geometry of 1–22 (E: C or N).[a]
Molecule
r1/r2 [æ]
a(E¢Cc¢E) [8]
Ref.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
1.519/1.311 (1.516/1.315) 1.524/1.325 1.515/1.326 1.405/1.359 1.419/1.375 1.404/1.387 1.422/1.355 1.529/1.362 1.533/1.380 1.522/1.380 1.414/1.415 1.425/1.425 1.409/1.433 1.345/1.345 (1.346) 1.401/1.350 1.404/1.404 1.371/1.371 (1.352/1.404) 1.429/1.376 1.439/1.439 1.360/1.360 (1.366/1.363) 1.408/1.368 1.418/1.418
106.3 103.6 101.8 105.2 101.2 98.3 99.9 105.0 102.5 100.6 104.0 99.9 97.1 115.6 116.0 116.3 110.8 112.4 111.5 114.9 115.4 115.7
[18]
(106.5)
As a first approximation, the stability of these molecules can be judged from their respective singlet–triplet energy gaps (DES–T).[12] In general, the stability of the singlet state increases with an increase in the value of DES–T. The calculated DES–T values are given in Table 2.
Table 2. PBE0/6-31 + G*-calculated singlet–triplet separations (DES–T) and stabilization energies (SE) of 1–22.
(114.6)
[32]
(108.4)
[33]
(114.4)
[34]
[a] Experimental values are given within parenthesis.
lengths. Similar results are obtained for 4–6, 8–10, and 11–13. Replacement of the two methyl groups attached to the carbon atom in the a position to the Cc atom in 1–3 by the ylide centers in 4–7 changes the Cc¢E bond lengths significantly. The Cc¢C (r1) bond lengths are found to decrease, whereas there is an increase in the Cc¢N (r2) bond lengths. This indicates an increase in electron delocalization between the formally vacant p orbital of the Cc atom and the adjacent carbon atom (Ca) but a decrease in delocalization from the nitrogen lone pair (NLP) to the formally vacant p orbital of the Cc atom. This is also evident from the increase in occupancy of the NLP in 4–7 (1.682, 1.578, 1.572, and 1.561, respectively) relative to that in 1–3 (1.551, 1.540, and 1.542, respectively). The E¢Cc¢E bond angles also change appreciably. There is an increase in the Cc¢E (both r1 and r2) bond lengths and a slight decrease in the E¢Cc¢E bond angles upon replacement of the amino groups of 1–3 with the ylide centers in 8–10. However, molecules 11–13, in which two ylide groups are introduced, were found to have smaller E¢Cc¢E bond angles than the parent compounds with identical backbones. Comparison of the Cc¢E bond lengths of 4–6 with those of 11–13 indicates that there is an increase in the Cc¢E (both r1 and r2) bond lengths as a result of replacement of the amino groups with ylide centers. However, there is a decrease in the r1 value and an increase in the r2 value upon replacement of the two methyl groups in 8–10 with the ylide centers in 11–13. In the case of the six-membered NHCs with a saturated backbone (14–16), the Cc¢E (both r1 and r2) bond lengths are found to increase upon replacement of the amino groups with ylide centers. Similar results are also obtained for boron-substituted five- (17–19) and six- (20–22) membered NHCs. However, in all three cases, the change in the central E¢Cc¢E bond angle is not significant. Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
Molecule
DES–T [kcal mol¢1]
SE [kcal mol¢1]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
43.9 40.3 39.6 55.3 59.4 63.5 61.7 40.4 37.1 36.8 57.5 59.9 64.2 58.0 50.3 50.4 43.6 35.3 31.7 42.2 39.6 40.6
79.4 72.6 69.8 103.6 114.5 114.6 113.9 84.7 77.3 74.9 105.8 114.3 115.8 101.9 97.7 107.3 82.3 86.9 90.4 87.7 86.4 95.8
It is evident from Table 2 that all of the molecules have a stable singlet ground state. The singlet state of 1 with a saturated backbone is slightly more stable than 2 with an unsaturated backbone. We did not observe any dramatic change in the stability of the singlet states as a result of the introduction of a nitrogen atom into the olefinic backbone. For example, the DES–T values of 2 and 3 (40.3 and 39.6 kcal mol¢1, respectively) and those of 9 and 10 (37.1 and 36.8 kcal mol¢1, respectively) are comparable. There is a significant increase in the DES–T values as a result of introduction of the more-electrondonating ylide groups (in 4–7) in place of the two methyl groups (in 1–3). On the other hand, replacement of the amino groups (in 1–3) by ylidic ones (in 8–10) decreases the DES–T values, albeit marginally. A comparison of the DES–T values of 8–10 with those of 11–13 shows that the introduction of one more ylide group dramatically enhances the stability. However, a comparison of 4–6 with 11–13 indicates that there is no significant increase in the DES–T values as a result of the replacement of the amine group with one more ylide group. Among the saturated six-membered NHCs (14–16), the parent carbene is found to have a higher DES–T value than those compounds with phosphorus ylide groups at the a position with respect to the carbene carbon atom. Similarly, lower singlet–triplet separation is obtained for both five- and six-membered boron-sub-
11605
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper stituted NHCs with ylide groups at the a position to the Cc atom relative to the parent compounds. The changes are more appreciable for the five-membered ones. The stabilization energies (SE) of 1–22 were evaluated by using Equation (1) and these values were further used to obtain a measure of the thermodynamic stabilities of all of the molecules. It may be noted that the calculated stabilization energies are the same as indirect hydrogenation energies. The calculated values of the stabilization energies are listed in Table 2.
Figure 1. Plot of the energies of the s-symmetric lone-pair orbitals (Es) concentrated on the central carbon atom of 1–22.
The stabilization energies of carbenes 1–13 follow an almost similar trend to the DES–T values. However, for molecules 14– 22, the stabilization energies do not go in parallel with the DES–T values. For example, the DES–T values of 17–19 are found to decrease as a result of the introduction of ylide groups, whereas the stabilization energies follow a reverse order. We obtained a reasonable correlation (R2 = 0.82, with omission of the point corresponding to 19; Figure S2 in the Supporting Information) between the DES–T and SE values and both of these values predict 13 to be the thermodynamically most stable molecule. The singlet–triplet separation and stabilization energies of 4–6 and 11–13 are found to be comparable with the theoretically modeled experimentally known carbenes 1 and 7. Similar comparisons can be made between the six-membered carbenes 15–16 and the boron-substituted carbenes 18–19 and 21–22 with the experimentally known carbenes 14, 17, and 20, respecively.
nating ability. A similar increase in electron-donating ability is obtained for six-membered NHCs with a saturated backbone (14–16) and for both five- (17–19) and six- (20–22) membered boron-substituted NHCs. Out of all of the molecules, 19 is found to have the highest s-donating ability. In order to get a better understanding of the s-donating ability of the carbenes considered for this study, we have analyzed the hybridization of the s-symmetric lone-pair orbital (LPs) at the Cc atom, along with the percentage of s-character (Table 3). The Es values increase with a decrease in the s-character of the lone pair at the Cc atom. Thus, molecules with lower s-character in the LPs are found to have better s-donating abilities. In fact, we obtained a good correlation (R2 = 0.79) between the energies and the percentage of s-character of the s-symmetric lone-pair orbitals at the Cc atom (Figure S3 in the
Ligand properties Both theoretical and experimental studies suggested that carbenes are very good s donors.[14, 15] The s-donating ability of carbene depends on the nature and energy of the s- symmetric lone-pair orbital (Es) concentrated at the Cc atom.[35] The higher the energy of this frontier orbital, the higher the donating ability will be. We have performed NBO[29] analysis in order to determine the energies of these s-donating frontier orbitals. The calculated Es values are listed in Table 3 and are graphically represented in Figure 1. Both Table 3 and Figure 1 indicate that there is a marginal decrease (except for 12) in the s-donating ability of carbenes 1–13 as a result of changes in the backbone. It is apparent from Figure 1 that replacement of the two methyl groups attached to the a-carbon atom with respect to the Cc atom in 1– 3 by the more-electron-releasing ylide groups (in 4–7) dramatically lifts the Es values and thereby significantly enhances the basicity of 4–7. Similarly, replacement of the amino groups of 1–3 with ylide groups (in 8–10) significantly increases the sdonating abilities. The introduction of two ylide centers near to the Cc atom (in 11–13) is found to further increase the s-doChem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
11606
Table 3. PBE0/6-31 + G*-calculated energies (Es), hybridization (% s character) of the s-symmetric lone-pair orbital concentrated at the carbene carbon atom and the average carbonyl-stretching frequencies (nCO) of L¢ Rh(CO)2Cl (L: 1–22) complexes.
Molecule
Es [eV]
Hybridization (% s character)
nCO (avg.) [cm¢1]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
¢5.5 ¢5.8 ¢6.2 ¢4.8 ¢5.0 ¢5.2 ¢5.3 ¢4.5 ¢4.6 ¢4.9 ¢4.1 ¢4.0 ¢4.2 ¢5.4 ¢4.3 ¢3.9 ¢5.3 ¢4.4 ¢3.5 ¢5.1 ¢4.3 ¢3.8
sp1.20 (45.4) sp1.09 (47.7) sp1.09 (47.8) sp1.48 (40.3) sp1.29 (43.7) sp1.26 (44.3) sp1.30 (43.5) sp1.70 (36.8) sp1.44 (41.0) sp1.44 (41.0) sp1.98 (33.5) sp1.68 (37.3) sp1.65 (37.7) sp1.23 (44.8) sp1.79 (35.8) sp2.60 (27.7) sp1.03 (49.1) sp1.49 (40.1) sp1.81 (35.6) sp1.11 (47.3) sp1.63 (37.9) sp2.25 (30.8)
2131 2136 2141 2120 2125 2129 2131 2122 2123 2129 2112 2112 2112 2136 2122 2110 2136 2122 2110 2131 2120 2101
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper ability (Table 3) and, consequently, the nCO value of the respective rhodium complex is found to be the highest. Similarly, for molecules like 16, 19, and 22, which are better s donors, the carbonyl-stretching frequencies are significantly lower than those of others. We have obtained a nice correlation (R2 = 0.88) between the Es and nCO values (Figure 3).
Figure 2. Correlation plot between the proton affinities [kcal mol¢1] and energies of the s-symmetric lone pair orbitals concentrated on the carbene carbon (Es [eV]) of 1–22.
Supporting Information). The basicity of these carbene molecules can also be gauged from an evaluation of their respective proton affinities.[36] The higher the basicity, the higher the proton affinity (PA) will be. Indeed, we obtained a nice correlation (R2 = 0.90) between the computed Es and PA values (Figure 2 and Table S3 in the Supporting Information). In agreement with their enhanced s-donating abilities, the PA values of the bis-ylide-containing carbenes are found to be significantly higher (> 300 kcal mol¢1) than others. The carbonyl-stretching frequencies (nCO) of the corresponding metal carbonyl complexes can be used as an important tool to measure the s-donating abilities of the carbene ligands.[37] The higher the s basicity of a carbene ligand, the higher the electron density at the metal center will be and, consequently, the higher the extent of back donation from the metal center to the CO antibonding orbital will be (Scheme 2), which will thereby shift the carbonyl-stretching frequencies (nCO) to lower values.
Figure 3. Correlation plot between the energies of the s-symmetric lonepair orbitals concentrated at the central carbene carbon atom (Es [eV]) and the nCO values [cm¢1] of the L¢Rh(CO)2Cl complexes (L: 1–22).
Nucleophilicity index The donor strength of all of the carbene molecules was further assessed by evaluating their nucleophilicity indices. The nucleophilicity index is calculated by using the equation N = EHOMO¢EHOMO(TCNE), with tetracyanoethylene (TCNE) considered as the reference.[38] The calculated values of the nucleophilicity indices are listed in Table 4.
Table 4. PBE0/6-31 + G*-calculated nucleophilicity indices (N) of 1–22.
Scheme 2. Schematic representation of the back-bonding interaction between the metal center and the CO p* orbital.
To get a quantitative measure of the electron-donating ability of the carbene ligands under consideration, we have optimized square-planar complexes of rhodium, L¢Rh(CO)2Cl (L: 1– 22) with the two carbonyl groups in the cis positions. The calculated values of nCO for all of the complexes are given in Table 3. The calculated average nCO values are found to vary as a function of the relative s basicity of the carbene ligands. The computed nCO values of the metal complexes containing ylidesubstituted carbenes are found to be appreciably lower than those of others. This indicates significantly enhanced electrondonating abilities of the ylide-containing carbenes. Among all of the molecules, 3 is found to have the lowest s-donating Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
Molecule
N
Molecule
N
1 2 3 4 5 6 7 8 9 10 11
4.1 3.7 3.4 4.8 4.6 4.4 4.3 5.1 5.0 4.7 5.9
12 13 14 15 16 17 18 19 20 21 22
5.6 5.4 4.2 5.3 5.9 4.3 5.2 6.1 4.5 5.3 5.8
The nucleophilicity indices of the carbene molecules are found to be in the range of 3.4–6.1 eV. These values show a similar trend to the respective Es values. The values of the nucleophilicity indices support the increase in s-donating ability of carbenes as a result of the introduction of ylide centers into the ring framework. The N values are found to be highest for carbenes with two ylide groups adjacent to the Cc atom. The highest and lowest s basicities are obtained for 19 and 3 (Table 3), respectively, which is in agreement with their respective N values. Indeed, we have obtained an excellent correlation (R2 = 0.98) between the Es and N values (Figure 4).
11607
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper Conclusion
Figure 4. Correlation plot between Es [eV] and N [eV] for 1–22.
Recently, Gandon and co-workers have shown that the nucleophilicity of a carbene ligand may be correlated with the degree of gallium pyramidalization (qGa ; qGa = 3608¢Cl¢Ga¢ Cl) in the carbene adducts of GaCl3.[39] By following this approach, we have calculated the GaCl3 adducts of 1–22 and obtained a good correlation (R2 = 0.85) between the nucleophilicity index and qGa values, as well as a nice correlation (R2 = 0.90) between the Ga¢Cc bond dissociation energies and bond lengths (Figure 5 and Table S3 in the Supporting Information). Furthermore, we have also obtained a nice correlation (R2 = 0.92) between the calculated proton affinity and the degree of Ga pyramidalization (Figure S4 in the Supporting Information).
Density functional theory calculations have been carried out on a number of cyclic carbene molecules with or without heteroatoms in the ring framework. All of these molecules exhibit a stable singlet ground state. The introduction of ylide centers into the ring framework was found to dramatically enhance the s-donating ability. Furthermore, for majority of the molecules, ylide substitution leads to a significant increase in stability. The carbonyl-stretching frequencies of the corresponding metal complexes and the nucleophilicity index values were found to correlate well with the s basicity of these carbenes. The calculated proton affinity values and the extent of Ga pyramidalization were also in excellent agreement with the s basicity of these carbenes. We hope that our study will trigger new experimental studies towards the exploration of novel ylide-stabilized carbenes.
Acknowledgements A.K.P. thanks Tezpur University for financial assistance and Prof. Holger Braunschweig for kindly granting access to the computational facilities of his research group. B.B. thanks Tezpur University for an institutional fellowship. Keywords: boron · carbenes · electron donation · rhodium · ylides
Figure 5. Correlation plots between: a) the pyramidalization angle at the gallium atom (qGa in degrees) of the GaCl3 adducts and the nucleophilicity indices (N), and b) the Ga¢Cc bond dissociation energies (BDE [kcal mol¢1]) and the Ga¢Cc bond lengths [æ] of the GaCl3 adducts of 1–22. Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
[1] a) K. Balasubramanian, A. D. McLean, J. Chem. Phys. 1986, 85, 5117; b) W. D. Allen, H. F. Schaefer, Chem. Phys. 1986, 108, 243. [2] A. J. Arduengo, III, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1991, 113, 361. [3] W. A. Herrmann, M. Elison, J. Fischer, C. Kçcher, G. R. J. Artus, Angew. Chem. Int. Ed. Engl. 1995, 34, 2371; Angew. Chem. 1995, 107, 2602. [4] D. Enders, O. Niemeier, A. Henseler, Chem. Rev. 2007, 107, 5606. [5] F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47, 3122; Angew. Chem. 2008, 120, 3166. [6] S. Dez-Gonzlez, N. Marion, S. P. Nolan, Chem. Rev. 2009, 109, 3612. [7] K. V. S. Ranganath, J. Kloesges, A. H. Schfer, F. Glorius, Angew. Chem. Int. Ed. 2010, 49, 7786; Angew. Chem. 2010, 122, 7952. [8] a) M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 2014, 510, 485; b) D. K. Roy, B. Mondal, R. S. Anju, S. Ghosh, Chem. Eur. J. 2015, 21, 3640. [9] A. J. Arduengo, III, H. V. R. Dias, R. L. Harlow, M. Kline, J. Am. Chem. Soc. 1992, 114, 5530. [10] A. J. Arduengo, III, J. R. Goerlich, W. J. Marshall, Liebigs Ann. 1997, 365. [11] D. Martin, A. Baceiredo, H. Gornitzka, W. W. Schoeller, G. Bertrand, Angew. Chem. Int. Ed. 2005, 44, 1700; Angew. Chem. 2005, 117, 1728. [12] a) D. Bourissou, O. Guerret, F. P. Gabbai, G. Bertrand, Chem. Rev. 2000, 100, 39; b) S. Gronert, J. R. Keeffe, R. A. More O’Ferrall, J. Am. Chem. Soc. 2011, 133, 3381. [13] V. Lavallo, Y. Canac, B. Donnadieu, W. W. Schoeller, G. Bertrand, Science 2006, 312, 722. [14] a) C. Boehme, G. Frenking, J. Am. Chem. Soc. 1996, 118, 2039; b) A. A. Tukov, A. T. Normand, M. S. Nechaev, Dalton Trans. 2009, 7015; c) P. Bazinet, G. A. P. Yap, D. S. Richeson, J. Am. Chem. Soc. 2003, 125, 13314; d) N. Kuhn, A. Al-Sheikh, Coord. Chem. Rev. 2005, 249, 829; e) V. Nair, S. Bindu, V. Sreekumar, Angew. Chem. Int. Ed. 2004, 43, 5130; Angew. Chem. 2004, 116, 5240. [15] a) M. Alcarazo, T. Stork, A. Anoop, W. Thiel, A. Fìrstner, Angew. Chem. Int. Ed. 2010, 49, 2542; Angew. Chem. 2010, 122, 2596; b) H. Seo, B. P. Roberts, K. A. Abboud, K. M. Merz, S. Hong, Org. Lett. 2010, 12, 4860;
11608
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper
[16] [17] [18]
[19] [20] [21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
c) T. W. Hudnall, C. W. Bielawski, J. Am. Chem. Soc. 2009, 131, 16039; d) R. Tonner, G. Heydenrych, G. Frenking, Chem. Asian J. 2007, 2, 1555. a) M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1, 953; b) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18. J. Huang, E. D. Stevens, S. P. Nolan, J. L. Petersen, J. Am. Chem. Soc. 1999, 121, 2674. a) V. Lavallo, Y. Canac, C. Prsang, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2005, 44, 5705; Angew. Chem. 2005, 117, 5851; b) V. Lavallo, Y. Canac, A. DeHope, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2005, 44, 7236; Angew. Chem. 2005, 117, 7402. M. Soleilhavoup, G. Bertrand, Acc. Chem. Res. 2015, 48, 256 and references therein. N. Holzmann, D. M. Andrada, G. Frenking, J. Organomet. Chem. 2015, DOI: 10.1016/j.jorganchem.2015.03.029. a) V. Lavallo, G. D. Frey, S. Kousar, B. Donnadieu, G. Bertrand, Proc. Natl. Acad. Sci. USA 2007, 104, 13569; b) G. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G. Bertrand, Science 2007, 316, 439. P. P. Samuel, K. C. Mondal, H. W. Roesky, M. Hermann, G. Frenking, S. Demeshko, F. Meyer, A. C. Stìckl, J. H. Christian, N. S. Dalal, L. Ungur, L. F. Chibotaru, K. Prçpper, A. Meents, B. Dittrich, Angew. Chem. Int. Ed. 2013, 52, 11817; Angew. Chem. 2013, 125, 12033. a) J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2007, 46, 7052; Angew. Chem. 2007, 119, 7182; b) J. D. Masuda, W. W. Schoeller, B. Donnadieu, G. Bertrand, J. Am. Chem. Soc. 2007, 129, 14180; c) O. Back, G. Kuchenbeiser, B. Donnadieu, G. Bertrand, Angew. Chem. Int. Ed. 2009, 48, 5530; Angew. Chem. 2009, 121, 5638. a) S. Nakafuji, J. Kobayashi, T. Kawashima, Angew. Chem. Int. Ed. 2008, 47, 1141; Angew. Chem. 2008, 120, 1157; b) M. Asay, B. Donnadieu, A. Baceiredo, M. Soleilhavoup, G. Bertrand, Inorg. Chem. 2008, 47, 3949; c) A. Fìrstner, M. Alcarazo, K. Radkowski, C. W. Lehmann, Angew. Chem. Int. Ed. 2008, 47, 8302; Angew. Chem. 2008, 120, 8426; d) J. Kobayashi, S. Nakafuji, A. Yatabe, T. Kawashima, Chem. Commun. 2008, 6233. a) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865; b) J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1997, 78, 1396; c) J. P. Perdew, K. Burke, M. Ernzerhof, J. Chem. Phys. 1996, 105, 9982; d) M. Ernzerhof, G. E. Scuseria, J. Chem. Phys. 1999, 110, 5029. a) M. Dolg, U. Wedig, H. Stoll, H. Preuss, J. Chem. Phys. 1987, 86, 866; b) D. Andrae, U. Hausserermann, M. Dolg, U. Wedig, H. Stoll, H. Preuss, Theor. Chim. Acta. 1990, 77, 123; c) A. Alkauskas, A. Baratoff, C. Bruder, J. Phys. Chem. A 2004, 108, 6863. a) A. K. Phukan, A. K. Guha, S. Sarmah, R. D. Dewhurst, J. Org. Chem. 2013, 78, 11032; b) A. K. Phukan, A. K. Guha, S. Sarmah, Organometallics 2013, 32, 3238; c) A. K. Guha, A. K. Phukan, J. Org. Chem. 2014, 79, 3830; d) B. Borthakur, T. Rahman, A. K. Phukan, J. Org. Chem. 2014, 79, 10801. a) A. A. Fokin, L. V. Chernish, P. A. Gunchenko, E. Y. Tikhonchuk, H. Hausmann, M. Serafin, J. E. P. Dahl, R. M. K. Carlson, P. R. Schreiner, J. Am. Chem. Soc. 2012, 134, 13641; b) B. D. Rekken, T. M. Brown, J. C. Fettinger, F. Lips, H. M. Tuononen, R. H. Herber, P. P. Power, J. Am. Chem. Soc. 2013, 135, 10134; c) C. Hering, A. Schulz, A. Villinger, Angew. Chem. Int. Ed. 2012, 51, 6241; Angew. Chem. 2012, 124, 6345.
Chem. Eur. J. 2015, 21, 11603 – 11609
www.chemeurj.org
[29] a) E. D. Glendening, A. E. Reed, J. E. Carpenter, F. Weinhold, NBO Program 3.1, W. T. Madison: 1988; b) A. E. Reed, F. Weinhold, L. A. Curtiss, Chem. Rev. 1988, 88, 899. [30] Gaussian 03, Revision D.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. J. Salvador, J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al- Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 2003. [31] The ylide-stabilized carbenes can be depicted by a resonance structure consisting of a double-bonded form, as shown in Scheme 1, and a zwitterionic form, in which positive and negative charges are placed at the phosphorus and carbon atoms, respectively. An example is shown below:
[32] M. Iglesias, D. J. Beetstra, J. C. Knight, L. Ooi, A. Stasch, S. Coles, L. Male, M. B. Hursthouse, K. J. Cavel, A. Dervisi, I. A. Fallis, Organometallics 2008, 27, 3279. [33] K. E. Krahulic, G. D. Enright, M. Parvez, R. Roesler, J. Am. Chem. Soc. 2005, 127, 4142. [34] C. Prsang, B. Donnadieu, G. Bertrand, J. Am. Chem. Soc. 2005, 127, 10182. [35] a) H. Jacobsen, A. Correa, C. Poater, C. Costabile, L. Cavallo, Coord. Chem. Rev. 2009, 253, 687; b) U. Radius, F. M. Bickelhaupt, Coord. Chem. Rev. 2009, 253, 678. [36] R. Tonner, G. Heydenrych, G. Frenking, ChemPhysChem 2008, 9, 1474. [37] a) A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller, R. H. Crabtree, Organometallics 2003, 22, 1663; b) D. G. Gusev, Organometallics 2009, 28, 763. [38] L. R. Domingo, E. Chamorro, P. Perez, J. Org. Chem. 2008, 73, 4615. [39] A. El-Hellani, J. Monot, S. Tang, R. Guillot, C. Bour, V. Gandon, Inorg. Chem. 2013, 52, 11493. Received: March 3, 2015 Published online on June 26, 2015
11609
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
